The present invention pertains generally to fluorine radical generators. More particularly, the present invention pertains to ion filters that are able to separate fluorine ions from a plasma. The present invention is particularly, but not exclusively, useful as a system for creating a plasma by sublimating a potassium fluoride source to create a plasma from which fluorine ions can be evacuated.
Fluorine (F) is a pale greenish yellow gas. It is the most electronegative (non-metallic) of the elements and is the first of the halogens, having only one electron vacancy in its outer energy level. Chemically, fluorine is highly corrosive and is a very reactive gas. Thus, it is never found free and, accordingly, its storage and delivery are problematic and raise serious safety implications. Moreover, fluorine gas is expensive.
Despite the difficulties that are confronted during the handling and use of fluorine, fluorine radicals have been found to be very useful in many different applications. For instance, NF3 has been used extensively in semiconductor applications. More specifically, NF3 has been used as a feed for etching semiconductors and, more recently, for the cleanup of vacuum chambers following an etching process. Many other examples, including other types of cleanup operations, can be cited wherein fluorine gas, or a fluorine radical, is useful.
It happens that potassium (K) reacts with fluorine (F) to create a potassium fluoride salt (KF). Unlike a fluorine radical, however, potassium fluoride salt (KF) is relatively inexpensive, and is more easily handled. Further, it also happens that a potassium fluoride salt (KF) is volatized into its constituents, K and F, when heated by a plasma. The resultant fluorine can then be used for the applications referred to above.
In light of the above, it is an object of the present invention to provide a fluorine generator that can be used as an xe2x80x9con-demandxe2x80x9d source of fluorine radicals. Another object of the present invention is to provide a fluorine generator that circumvents the need for implementing the safety procedures that are required for the long-term storage of fluorine. Still another object of the present invention is to provide a convenient source of relatively clean, uncontaminated fluorine. Yet another object of the present invention is to provide a fluorine generator that is simple to use, is relatively easy to manufacture, and is comparatively cost effective.
In accordance with the present invention, a fluorine generator includes a generally cylindrical shaped vacuum chamber having a first end and a second end. The chamber defines a longitudinal axis and it is filled with a working gas. For purposes of the present invention this working gas is preferably either Neon (Ne) or Nitrogen (N2).
A dielectric window is located at the first end of the chamber, and a potassium fluoride (KF) source is positioned in the chamber adjacent the window. Preferably, the KF source will be a flat, generally annular shaped disk made of a potassium fluoride salt. An r-f (radio frequency) antenna is positioned outside the chamber with the dielectric window located between the KF source and the r-f antenna. Further, the generator includes a generally annular shaped collector that is positioned inside the chamber between said first and second ends, and at a distance xe2x80x9caxe2x80x9d from the axis.
In further detail, the generator includes a plurality of concentric electrodes that are mounted inside the chamber. Specifically, these electrodes are centered on the axis between the KF source and the second end of the chamber, and they are used to create a radially oriented electric field, Er, in the chamber. Importantly, the electrodes are biased to create a parabolic profile of electric potential for the electric field, Er, that can be defined as xcfx86¢(r)=U(1xe2x88x92r2/a2). In this case, xe2x80x9cUxe2x80x9d is the voltage on the axis (which will preferably be below 200 volts) and xe2x80x9crxe2x80x9d is a radial distance from the axis. Additionally, the generator includes magnetic coils that are mounted on the outside of the chamber to create an axially oriented and substantially uniform magnetic field, B, inside the chamber. Thus, the electric and magnetic fields are crossed (Erxc3x97B).
In the operation of the generator, the working gas is first introduced into the chamber. The r-f antenna is then activated to heat the working gas. This heating ionizes the working gas and, in turn, causes the KF source to sublimate. The result is the creation of a plasma in the chamber that includes potassium ions, fluorine atoms, fluorine ions and working gas ions. It is an important aspect of the present invention that the crossed electric and magnetic fields (Erxc3x97B) are controlled to drive the potassium ions on unconfined orbits toward the collector inside the chamber. Thus, the potassium ions, which have a mass weight of 39 and are therefore heavier than the ions of either the working gas or of fluorine, are separated from the plasma. Specifically, by being on unconfined orbits, the potassium ions do not exit the chamber and are, instead, collected on the collector in the chamber. On the other hand, contrary to its effect on the potassium ions, the controlled (Erxc3x97B) is established to place the fluorine ions and the working gas ions on confined orbits around the axis. Thus, these ions are directed toward the second end of the chamber. These ions are then evacuated from the chamber through its second end for the collection of the fluorine.
An important consideration for the operation of the generator of the present invention is the maintenance of a pressure of approximately 1-10 mTorr in the chamber. This is done by concerted control over the introduction of the working gas into the chamber and the evacuation of fluorine and working gas ions from the chamber. A consequence here is that the potassium ions, fluorine ions and working gas ions have respective densities in the plasma, wherein the densities of the potassium and fluorine ions are in a range of 5-20% of the density of the working gas ions.